Accounting for Climate in Ranking Countries’ Carbon Dioxide Emissions

Bringing In Climate

The approach we recommend involves adjusting each country’s emissions by the combined demand for heating and cooling based purely on climatology, not on the actual methods employed for changing temperature. Specifically, this approach involves adjusting the amount of emissions in accordance with the total number of heating degree days and cooling degree days, which allows us to calculate the amount of emissions per heating and cooling degree day. Heating degree days offer an index of the energy demand required to heat indoor spaces. This index is calculated by subtracting the mean daily outdoor temperature from 18 degrees Celsius and summing up only positive values over a fixed period, such as an entire year. Cooling degree days represent an analogous index of the energy demand for cooling.

For an average outdoor temperature lower than 18 degrees, most buildings in developed countries require heating to maintain a 21-degree indoor temperature. Conversely, for an average outdoor temperature higher than 18 degrees, most buildings require cooling to maintain a 21-degree indoor temperature. The selection of 18 degrees as the base outdoor temperature accounts for the additional heat generated by occupants and their activities, resulting in an average indoor temperature of 21 degrees at 18 degrees outdoors.

Heating and cooling degree days, being based solely on temperature, do not take into account secondary variables influencing the need for heating and cooling (such as humidity and cloud cover). Nonetheless, studies have shown that energy consumption is highly correlated with degree days for both heating and cooling. Total heating and cooling degree days is a combined index of heating degree and cooling degree days. (This index does not incorporate potential differences in the energy expenditures required to heat or to cool by one degree.)

Let’s consider an example. Country A and Country B have the same total emissions per capita and GDP. However, Country A is located in a relatively extreme climate (either very cold or very hot), and Country B is located in a mild climate. Country A thus has more population-weighted heating and cooling degree days (taking into account where within the country people live) than does Country B. Under traditional measures, the two countries would have the same ranking. But our intuitions recognize Country A as a much more efficient emitter—and this difference is reflected in the new measure.

No one-to-one correspondence can be made between actual emissions and demand based on climate. First, technologies for cooling indoor spaces, such as air conditioning, are not yet universally accessible. Second, the set point (the thermostat setting for the indoor temperature) varies between individuals and between countries. Third, the various options for climate control that are in use—heating by oil, natural gas, electricity and geothermal energy, for instance—have different consequences for carbon dioxide emissions. Our proposed approach bypasses all of these considerations by providing an adjustment based purely on climatology.